A MOS driver circuit can comprise a first output transistor and a second output transistor which are driven in push-pull into a conducting or nonconducting state by a first driver stage and a second driver stage, respectively, and which are interconnected at a junction point to which a load to be driven is connectable, with a first hold stage connected between a data input and the first driver stage, and a second hold stage connected between the data input and the second driver stage, the first hold stage transferring a data signal coming from the data input to the first driver stage in response to an enable signal provided by the second driver stage, and the second hold stage transferring the data signal coming from the data input to the second driver stage in response to an enable signal provided by the first driver stage.
Such a driver circuit is disclosed in co-pending German Patent Application Number DE-C 3708499, assigned to SGS Halbleiter-Beuelemente GmbH filed in the names of the inventors Hans Reichmeyer and Josef Stockinger. It is typically used as an output stage of logic circuits in CMOS technology which serves to drive loads. In such driver circuits, steep pulse edges must be prevented, since they may cause interference because of their high frequency components. Therefore, the output transistors of the driver stages are driven into the conducting or nonconducting state such that the transition is continuous, i.e., not rectangular, but with a distinctly limited edge steepness. To accomplish this, high-impedance driver stages are used.
Such a driver circuit has the disadvantage that when noise-loaded outputs are driven, a shunt current may flow between the output transistors while the output transistors are being held in the conducting or nonconducting state. This is due to the fact that in the presence of external interference, the voltage at the drain of the output transistor which is in the nonconducting state will change. Since there is always a parasitic capacitance between the drain and the gate of the transistor, the gate voltage of the transistor will also change. This voltage change may cause the transistor to be switched into a conducting state, so that a shunt current will flow between the transistors. External interference can reach an output, e.g., through capacitive and inductive coupling of adjacent signals which are present, for example, in the integrated circuit package, on the circuit board, or in a cable harness. Such shunt currents cause unnecessary power dissipation and unnecessary electromagnetic interference.
It is the object of the invention to provide a MOS driver circuit in which any shunt currents between the output transistors are prevented when driving resistive, capacitive, and inductive loads.